1. Field of the Invention
The present invention relates to voltage to current converters, i.e., circuits that produce an output current as a linear function of an input voltage.
2. State of the Art
In numerous circuit applications, the working medium of the circuit is current whereas the medium used to control the circuit is voltage. Such applications require the use of a voltage to current converter in which a voltage signal is converted to a corresponding current signal in a linear fashion. One such application is a phase locked loop in which a current controlled oscillator is made to be voltage controlled using a voltage to current converter. Phase locked loops are used widely in modern electronic devices of all types.
Referring to FIG. 1, in a conventional voltage to current converter, an input voltage V.sub.in is buffered using a buffer amplifier and impressed across a resistor R, a linear element, to produce a current I that is linearly related to the voltage V.sub.in. More particularly, the voltage V.sub.in is connected to the positive input of an op-amp 11. The output of the op-amp is connected to the gate electrode of a MOSFET MN1. The source electrode of the MOSFET MN1 is connected to the negative input of the op-amp 11, forming a negative feedback loop. The resistor R is connected between the source electrode of the MOSFET MN1 and ground. Two additional MOSFETs MP1 and MP2 are connected to form a current mirror that produces two substantially equal currents.
The negative-feedback op-amp 11 drives the MOSFET MN1 such that the voltage at the negative input of the op-amp 11 equals the voltage at the positive input of the op-amp 11, namely V.sub.in. This occurs when the MOSFET MN1 conducts sufficiently to allow a current I=V.sub.in /R to flow from the MOSFET MN1 through the resistor R. An identical current I is produced in the MOSFET MP2 by action of the current mirror. This current is the output current of the voltage to current converter, and might be used to charge a capacitor C, for example. In a phase locked loop, R and C might form part of an oscillator controlled by the current I.
Although the voltage to current converter of FIG. 1 is highly linear in its operation, the trend in present-day MOS circuits is toward all-digital circuits. The analog resistor prevents the voltage to current converter from being realized in an all-digital fashion. Furthermore, the analog resistor does not experience the same process variations as the digital portion of a circuit the voltage to current converter forms a part of. As a result of the lack of tracking between the analog resistor and the digital transistors in the larger circuit, the actual behavior of the circuit may vary from its expected behavior.
For example, in a phase locked loop, because of process variations, the digital transistors in a pre-divider circuit might be "weak", causing operation of the pre-divider to be slowed. Because the analog resistor is not subject to the same process variations, the maximum frequency of the voltage controlled oscillator of the phase locked loop will remain the same. When the frequency of the voltage controlled oscillator exceeds the maximum frequency of the pre-divider, a lockup condition occurs in which the phase locked loop "goes open loop" and cannot be brought back into its normal operation range.